My understanding was that slavery, while legal in the North at the time of the early republic, was more or less dying out up there and was gradually replaced with a more capitalist, wage-driven form of labor rather than the large planters and small, yeomen farms of the South. That might be completely wrong, but it was the impression I got from reading about the era.
“My understanding was that slavery, while legal in the North at the time of the early republic . . .”
Not just legal; people in northern states owned and worked slaves. And the northern states voted to include slavery into the United States Constitution - probably because they thought it was in their own political and economic best self interests.
The author of the article - who goes by the name Gen Z - likely did not know those facts; and that’s the reason I mention them.